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Modelling Chemical and Physical Processes of Wood and Biomass Pyrolysis
Abstract
This review reports the state of the art in modeling chemical and physical processes of wood and biomass pyrolysis. Chemical kinetics are critically discussed in relation to primary reactions, described by one- and multi-component (or one- and multi-stage) mechanisms, and secondary reactions of tar cracking and polymerization. A mention is also made of distributed activation energy models and detailed mechanisms which try to take into account the formation of single gaseous or liquid (tar) species. Different approaches used in the transport models are presented at both the level of single particle and reactor, together with the main achievements of numerical simulations. Finally, critical issues which require further investigation are indicated.
... This has led to the formulation of many global models (single-step and multi-step methods) regarding the conversion of biomass to char, bio-oil, volatiles, etc. Some of the global models that are widely used are shown in Table 2. Owing to the simplistic nature of these models and the ease of making kinetic predictions, the global models were explored widely by the researchers [49][50][51][52]. However, the reliability of the kinetic triplet obtained from these models was questionable. ...
... where k 2 , k 3 , and k 4 are the kinetic rate constants for the reactions obtained by the reaction kinetic studies [49,63]. The liquid fraction of condensable gases has also been considered in some models as an additional model component. ...
... where Q s is the energy source term due to the pyrolysis reaction, and it is expressed in terms of the specific heat of the pyrolysis reaction. For the fluid (volatile gas/condensed liquid) domain, the energy equation is as follows [32,49]: ...
Pyrolysis, a process for extracting valuable chemicals from waste materials, leverages computational fluid dynamics (CFD) to optimize reactor parameters, thereby enhancing product quality and process efficiency. This review aims to understand the application of CFD in pyrolysis. Initially, the need for pyrolysis and its role in biomass valorization are discussed, and this is followed by an elaboration of the fundamentals of CFD studies in terms of their application to the pyrolysis process. The various CFD simulations and models used to understand product formation are also explained. Pyrolysis is conducted using both conventional and microwave-assisted pyrolysis platforms. Hence, the reaction kinetics, governing model equations, and laws are discussed in the conventional pyrolysis section. In the microwave-assisted pyrolysis section, the importance of wavelength, penetration depth, and microwave conversion efficiencies on the CFD are discussed. This review provides valuable insights to academic researchers on the application of CFD in pyrolysis systems. The modeling of pyrolysis by computational fluid dynamics (CFD) is a complex process due to the implementation of multiple reaction kinetics and physics, high computational cost, and reactor design. These challenges in the modeling of the pyrolysis process are discussed in this paper. Significant solutions that have been used to overcome the challenges are also provided with potential areas of research and development in the future of CFD in pyrolysis.
... 3) The scheme of coal pyrolysis is stepwise with parallel reactions [52]. 4) A single-group approximation was adopted when describing the process of radiation energy transfer. ...
... This is due to the fact [61] when modeling thermal decomposition processes, the use of one or another kinetic scheme has virtually no effect on the average integral thermal effect of the pyrolysis process. It must also be said that when mathematically modeling the pyrolysis processes of any organic fuel, the choice of the kinetic scheme (complex multi-stage or single-stage) in principle does not affect the composition of the thermal decomposition products, which, as a rule, is known from the results of specially conducted experiments (for example [52]). ...
... Although modern boilers operate with oxygen-limited combustion under a low primary air flow rate, it is important to point out that most of the time this equipment operates with reaction-limited combustion due to a high primary air supply [56]. However, most of the studies in the literature have investigated pyrolysis using inert atmospheres [40,44,46,47,[57][58][59][60][61][62][63][64].This is due to the fact that pyrolysis is the first step in thermochemical processes such as combustion and gasification [64]. ...
... Although modern boilers operate with oxygen-limited combustion under a low primary air flow rate, it is important to point out that most of the time this equipment operates with reaction-limited combustion due to a high primary air supply [56]. However, most of the studies in the literature have investigated pyrolysis using inert atmospheres [40,44,46,47,[57][58][59][60][61][62][63][64].This is due to the fact that pyrolysis is the first step in thermochemical processes such as combustion and gasification [64]. ...
Driven by its accessibility, extensive availability, and growing environmental consciousness, solid biomass has emerged as a viable alternative to enhance the diversity of renewable energy sources for electricity generation. To understand the phenomena involved in solid biomass conversion, it is necessary not only to understand the stages of the biomass combustion process but also to understand specifically the kinetics of the reaction and the release of the volatiles. The present work presents an overview of the existing literature on several topics related to the biomass combustion process, its characterization, as well as strategies to develop simple and effective models to describe biomass conversion with a view to the future development of numerical simulation models. Since the focus of most of the investigations is the development of a numerical model, a summary and identification of the different model assumptions and problems involved in thermal analysis experiments are presented. This literature review establishes the significance and credibility of the research, providing the main concepts and assumptions with a critique on their validity. Hence, this work provides specific contributions from a multi-scale perspective which can further be extended to provide insights into the design and optimization of biomass combustion technologies, such as boilers and furnaces.
... The second region, in the high temperature rage (T > 673 K) corresponds to char combustion and pyrolysis of the more stable biomass fractions, mainly lignin, whose degradation is known to span over a broad temperature range (433-923 K circa). Mineral matter transformation can also occur in the high temperature [3] Oxy-fuel carbon capture technology for pulverized fuel boilers [3] Anca-Couce [4] Reaction mechanisms and modelling of pyrolysis [4] Babu [5] Biofuels, bioproducts and biorefining [5] Cai et al. (2018) Modeling of ash formation and deposition in PF 1 fired boilers [6] Choi et al. [7] Kinetic modeling and CFD 2 [7] Dernbecher et al. [8] Application of CFD 2 in modeling biomass combustion systems [8] Di Blasi [10] Modeling chemical and physical processes of pyrolysis [9] Di Blasi [9] Combustion and gasification rates [10] Fatehi et al. [11] Modeling and optical studies of single particle combustion [11] Haberle et al. [12] Numerical models for biomass particles for domestic appliances [12] Hosseini et al. (2019) Fixed bed combustion modeling [13] Kleinhans et al. [14] Ash formation and deposition [14] Leng et al. [15] Nitrogen transformation [15] Lewandowski et al. [16] Thermal biomass conversion [16] Mazaheri et al. [17] Numerical simulation of biomass gasification [17] Miao et al. [18] Modeling biomass gasification in circulating fluidized beds. ...
... The second region, in the high temperature rage (T > 673 K) corresponds to char combustion and pyrolysis of the more stable biomass fractions, mainly lignin, whose degradation is known to span over a broad temperature range (433-923 K circa). Mineral matter transformation can also occur in the high temperature [3] Oxy-fuel carbon capture technology for pulverized fuel boilers [3] Anca-Couce [4] Reaction mechanisms and modelling of pyrolysis [4] Babu [5] Biofuels, bioproducts and biorefining [5] Cai et al. (2018) Modeling of ash formation and deposition in PF 1 fired boilers [6] Choi et al. [7] Kinetic modeling and CFD 2 [7] Dernbecher et al. [8] Application of CFD 2 in modeling biomass combustion systems [8] Di Blasi [10] Modeling chemical and physical processes of pyrolysis [9] Di Blasi [9] Combustion and gasification rates [10] Fatehi et al. [11] Modeling and optical studies of single particle combustion [11] Haberle et al. [12] Numerical models for biomass particles for domestic appliances [12] Hosseini et al. (2019) Fixed bed combustion modeling [13] Kleinhans et al. [14] Ash formation and deposition [14] Leng et al. [15] Nitrogen transformation [15] Lewandowski et al. [16] Thermal biomass conversion [16] Mazaheri et al. [17] Numerical simulation of biomass gasification [17] Miao et al. [18] Modeling biomass gasification in circulating fluidized beds. ...
... The pyrolysis kinetics of cellulose, hemicellulose, and lignin have been widely studied and are commonly considered pseudo-components of biomass components (Anca-Couce et al. 2014;Di 2008). Table 2 summarizes the activation energies of biomass pyrolysis using different pseudo-component and kinetic methods. ...
... E was reported to be in the range of 31.31-80 kg/mol for the hemicellulose pseudo-component, while greater values were reported by Chen et al. (2020) and Di (2008). As lignin has a significant amount of highly cross-linked polyphenolic aromatic ring polymers. ...
Analyzing the kinetic behavior of sewage sludge pyrolysis is essential for the design of efficient reactors to produce biofuel and syngas. To understand the complex pyrolysis process of sewage sludge, we pyrolyzed six model components (i.e., cellulose, hemicellulose, lignin, protein, soluble sugars, and lipid) using a thermogravimetric analyzer. The effects of the heating rate on the pyrolysis process were examined at four different heating rates (5, 15, 25, and 50 °C/min). As temperature increased, the derivative thermogravimetric peaks shifted to higher temperature zones. The temperature ranges of the maximum mass loss rate for cellulose, hemicellulose, lignin, protein, soluble sugars, and lipid were within 326.1–368.0 °C, 288.7–315.5 °C, 375.1–429.4 °C, 291.9–308.0 °C, 251.0–314.1 °C, and 410.8–454.1 °C, respectively. The apparent activation energies of the model components were obtained using non-isothermal kinetic analysis methods (Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose). In addition, a back-propagation artificial neural network with a momentum algorithm (BPM) was developed to predict the relationship between the pyrolysis experiment and the activation value. The best BPM model (BPM5) for predicting the cellulose pyrolysis was identified.
... Pyrolysis results in dehydration, followed by a decarboxylation, and, at highest temperatures, aromatisation during the carbonisation [66]. Small molecular carbohydrates are the first to decompose at 200-300 °C, whilst most cellulose and lignin are pyrolysed in the temperature range of 300-350 °C and protein and lipid in the temperature range of 200-600 °C. ...
Thermochemical conversion of sewage sludge was proven as a useful method for waste management. Moderate temperatures in the absence of oxygen (pyrolysis) and hydrothermal carbonisation (HTC) performed in the presence of water, under autogenous pressures, were selected to treat the same sample of anaerobic-digested sewage sludge (SS). Two hydrochars were prepared by HTC in one high-pressure reactor using SS at 80% humidity content and heating it at 180 and 240 °C for 4 h, leading to H180-4 and H240-4, respectively. Two pyrochars were prepared from a pre-oven-dried SS at 105 °C for 48 h, followed by slow pyrolysis at 300 °C for 1 h, and 400 °C for 1 h leading to P300-1 and P400-1, respectively. HTC and slow pyrolysis significantly increased thermal stability of chars with higher temperatures, only reducing organic matter content (from 68.4 to 46.7–59.2%). Based on the characterisation results, the treatments could be a suitable choice to pre-treat sludge as soil amendment. Higher temperatures of pyrolysis would be attractive to store stable carbon in soil and construction materials, whilst lower temperatures of pyrolysis and HTC would produce a material that could be used as a source of organic matter providing a pool of labile carbon and fixed carbon. Thermochemical conversions generated mesopores (10–50 nm, >40%) and slightly increased surface area from 2 m2/g (SS) to 5–17 m2/g (pyrochars) and to 20 m2/g (hydrochars). Hence, HTC showed the greatest potential to produce a material with higher adsorption capacity (100 cm3/g for H180-4) but all chars should be subjected to an activation process to be able to compete with other kinds of feedstocks. The reduction of the H:C and O:C from the original SS after the treatments indicated a greater carbonisation degree, but a general reduction of the high heating value (HHV) from 17.94 MJ kg−1 in SS to (14.93 MJ kg−1). The torrefied char and hydrochars could be an attractive option to reduce energy of the process and drying stage in the case of HTC.
... Typically, the resulting product composition includes bio-crude oil, H 2 O, and non-condensable gas (CH 4 , CO, CO 2 , H 2 ) with the calculations conforming to the law of mass conservations [43]. Kinetic studies aimed at understanding the formation of volatile products involved the assumption of lumped-kinetic model based on biomass structure, which is categorized as cellulose, hemicellulose, and lignin [44,45]. It should be noted that the lumped-kinetic scheme remains complex, predominantly addressing the thermal degradation behavior of separate components of biomass rather than comprehensively considering the whole lignocellulosic structure [46]. ...
The investigation of natural rubber pyrolysis holds substantial significance for the advancement of renewable hydrocarbon chemical production. Given its composition of almost 100% volatile matter and nearly zero fixed carbon content, natural rubber as a renewable chemical source offers a promising route for bio-hydrocarbon producers. Remarkably, natural rubber pyrolysis behavior and kinetic mechanism still have received limited attention. This study, hence, aims to characterize and model natural rubber pyrolysis, which offers a better understanding of the pyrolysis phenomenon. The modeling employs a novel methodology, the volatile state approach, as the basis for kinetic investigation to comprehensively predict the mass composition and yield, as well as determine kinetic parameters. The pyrolysis was investigated under variations of temperature conditions in laboratory-scale of semi-batch thermogravimetry apparatus. The kinetic parameters were obtained from the combination of Arrhenius equation and volatile state Kissinger-Akahira-Sunose (KAS) method. The result reports that the pyrolysis of natural rubber presents excellent potential for producing a range of valuable products, including both liquid and gas products as bio-hydrocarbon (CH4, C2H4, C2H6, C3H6, C3H8, C4H10, and C5H12) and non-hydrocarbon (CO2, CO, and H2). Moreover, the activation energy of each gas component is obtained as temperature dependence while still adhering to the mass conservation principle. Notably, the volatile state kinetic modeling is valuable for enhancing the understanding of natural rubber pyrolysis, providing kinetic parameters for pyrolysis optimization and prediction of the composition and yield of each gas product, closely aligned to the experimental data.
... Dünya üzerinde yaklaşık olarak yıllık 1.5 milyar biyokütle malzemesi üretilmektedir. Biyokütlenin yakıt olarak kullanılması ile içeriğindeki düşük sülfür (S) ve Nitrojen (N) nedeniyle CO2, SOx ve NOx oluşumuna düşük oranda etki etmektedir (Blasi, 2008-Lou et al., 2010 Odun, biyokütle kaynağı olarak yaygın bulunan bir materyaldir ve yeni teknolojik proseslerde kaynak olarak kullanılmaktadır (Guss, 1995 (Naik et al., 2010). Piroliz, oksijensiz ortamda biyokütlenin termal olarak parçalanarak katı, sıvı ve gaz ürün elde edilen termokimyasal dönüşüm yöntemlerinden bir tanesi olarak bilinmektedir. ...
... It has been estimated that a major part of the renewable energy can be generated from biomass. Thermochemical conversion of solid biomass (torrefaction, pyrolysis, gasification, and hydrothermal liquefaction) is considered prominent route for conversion of biomass to renewable fuels like producer gas, biooil, and biochar [1][2][3]. Gasification is the process to generate producer gas from biomass in which ligno-cellulosic biomass reacts with air at high temperature. Producer gas generated by gasification of biomass is suitable for electricity production, thermal application, and even for small-scale application like refrigeration and space heating [4,5]. ...
Tar in producer gas causes detrimental effects on downstream pipelines in gasifier-based power generation systems. Char-supported Ni-based catalysts have been proven to be effective and low-cost solution for tar reduction. Experiments have been conducted with chars derived from various types raw materials available onsite or offsite. This paper investigates the influence of char derived from pigeon pea stalk on the tar reduction of producer gas generated from pigeon pea stalk pellets. The effects of char bed temperature, Ni loading, particle size of char, and gas residence time have also been studied. A laboratory scale downdraft gasifier was used for gasification and an ex situ reactor was used for tar reduction experiments. Taguchi method was used for the optimization analysis of the variables affecting tar reduction efficiency. Experiments showed significant impact of temperature and Ni loading and lesser impact of particle size and gas residence time on reduction of tar in producer gas. Optimum values of operating parameters were 700°C bed temperature, 0.4 mole Ni loading, 1–5 mm char particle size, and 8-s gas residence time. The gas yield due to catalyst activity was enhanced by 3.8%, which further caused an improvement in the gas calorific value by 15.2% and cold gas efficiency by 10.8%. The catalyst was found to be active after 8 h of operation giving tar reduction of 81% at 700°C temperature.
Graphical abstract
... In the second phase, a kinetic reaction model is implemented for the primary pyrolysis reactions. It is an interlinked model of individual decomposition reactions of cellulose, hemicellulose, and lignin, according to [61,113]. The reactor type can be chosen according to the pyrolysis reactor that needs to be modeled. ...
Biomass gasification has obtained great interest over the last few decades as an effective and trustable technology to produce energy and fuels with net-zero carbon emissions. Moreover, using biomass waste as feedstock enables the recycling of organic wastes and contributing to circular economy goals, thus reducing the environmental impacts of waste management. Even though many studies have already been carried out, this kind of process must still be investigated and optimized, with the final aim of developing industrial plants for different applications, from hydrogen production to net-negative emission strategies. Modeling and development of process simulations became an important tool to investigate the chemical and physical behavior of plants, allowing raw optimization of the process and defining heat and material balances of plants, as well as defining optimal geometrical parameters with cost- and time-effective approaches. The present review paper focuses on the main literature models developed until now to describe the biomass gasification process, and in particular on kinetic models, thermodynamic models, and computational fluid dynamic models. The aim of this study is to point out the strengths and the weakness of those models, comparing them and indicating in which situation it is better to use one approach instead of another. Moreover, theoretical shortcut models and software simulations not explicitly addressed by prior reviews are taken into account. For researchers and designers, this review provides a detailed methodology characterization as a guide to develop innovative studies or projects.
... The δ 13 C of carbonized wood, especially archaeological charcoals, which are better preserved than wood in sediments over time, has been used to investigate paleoclimate [14][15][16][17][18]. Charcoals result from the incomplete combustion of woody material under an O 2 -free or -restricted atmosphere [19,20]. Wildfires as well as anthropogenic fires cause heterogeneous charring of woody material with fast-burning high-temperature areas on the wood surface and smouldering lower-temperature regions in the interior [21,22]. ...
The carbonization process induces significant physical, elemental, and structural transformations of wood. In this study, the modification of δ18O in wood during the carbonization process was investigated in conjunction with elemental analysis, infrared spectroscopy (FTIR), and Rock-Eval thermal analysis, to explore the connection between the chemical composition of the materials and the alterations in δ18O. Quercus petraea wood samples were experimentally burned at temperatures ranging from 200 °C to 1000 °C under inert (pyrolysis) and oxidative atmospheres. The results reveal that the modification of δ18O values in charred wood can be described as a sequential two-step process. The initial step, occurring below 300 °C, involves the volatilization and preferential degradation of thermolabile compounds, leading to an increase of +1.6‰ in δ18O. The subsequent step, below 700 °C, results in a decrease in δ18O values of –21.6‰, primarily driven by the thermal degradation of cellulose and lignin, as well as the increase of aromaticity and reorganization. The components and the δ18O of wood undergo distinct changes in combustion mode, due to different carbonization kinetics as evidenced by FTIR and elemental analysis. To assess the intensity of the carbonization process, influenced by temperature, oxygen availability, and wood characteristics, the H/C atomic ratio, a good indicator of aromaticity, is used. A non-linear regression model was established, relating δ18O to the H/C atomic ratio, thereby demonstrating that δ18O values undergo changes as wood aromatization progresses, independent of the carbonization conditions. The second order model has a mean confidence interval of 1.9‰ and a prediction interval of 8.1‰. This work provides a fundamental understanding of the connection between the chemical composition of woody materials, alterations in δ18O, and the carbonization process, offering valuable insights for further studies and applications related to oxygen-related information that may be preserved in charcoals.
... At this stage, the pyrolysis reaction of the lignin continued, and the DSC curve continued to fall. This outcome shows that there was a certain amount of crude fat and crude protein present in the sweet cherry leaves that can only be thermally degraded at high temperatures (Di Blasi 2008;Patwardhan et al. 2009). The pyrolysis process of biomass materials in these four stages can be analyzed and confirmed through various experimental equipment and methods. ...
Differences in the pyrolysis characteristics of leaves of sweet cherry tree (Prunus avium L.) under rain-shelter cultivation (RS) or under open-field cultivation (CK) were analysed using thermogravimetry (TG), derivative thermogravimetry (DTG), and differential scanning calorimetry (DSC) at three heating rates of 10, 20, and 30 °C·min-1. There were two obvious mass loss peaks at 280 °C and 330 °C, which were manifested by the slow pyrolysis of hemicellulose in the low temperature region and the rapid pyrolysis of cellulose in the high temperature region, respectively. The curve in the pyrolysis range after 440 °C was stable, and the mass change corresponded to the pyrolysis of a small amount of macromolecular organic extracts and inorganic salts. When the temperature reached 600 °C, approximately 69% and 73% of the CK and RS leaves were thermally destroyed, respectively. The Coats-Redfern method was used for kinetic calculations to obtain an activation energy of 29.8 to 36.1 kJ·mol-1 in the first-order pyrolysis kinetics stage. The second-order pyrolysis kinetics stage can fit the pyrolysis process well. A significant difference was observed in the pyrolysis characteristics or the kinetics between CK and RS, which were related to the heating rate and the hemicellulose content, cellulose content, and lignin ratio in each sample.
... Several recent studies have promoted the use of isoconversional methods to assess the chemical reactivity (e.g., with the determination of isoconversional activation energy) of a complex chemical process without assuming any form for the reaction model [15,[26][27][28][29]. Meaningful mechanistic insights can be drawn from those trends [29,30]. Yet, the overall prediction of the biomass pyrolysis process requires an explicit mathematical form for both intertwined chemical and physical phenomena [31]. ...
... In the second phase, a kinetic reaction model is implemented for the primary pyrolysis reactions. It is an interlinked model of individual decomposition reactions of cellulose, hemicellulose and lignin, according to [62,107]. The reactor type can be chosen according to the pyrolysis reactor that wants to be modelled. ...
Biomass gasification acquired great interest over the past decades as an effective and trustable technology to produce energy and fuels with net-zero carbon emissions. Moreover, using biomass waste as feedstock enables to recycle organic wastes and to fit the circular economy goals thus reducing the environmental impacts of waste management. Even if many studies have been already carried on, this kind of process must still be investigated and optimized with the final aim to develop industrial plants for different applications, from the hydrogen production to net-negative emission strategies. Modeling and developing of process simulations became an important tool to investigate the chemical and physical behavior of the plant, allowing to make a first raw optimization of the process and to define heat and material balances of the plant as well as to define optimal geometrical parameters with cost and time effective approaches. The present review paper focuses on the main literature models developed until now to describe biomass gasification process, and in particular on kinetic models, thermodynamic models, and computational fluid dynamic models. The aim of this study is to point out the strengths and the weakness of those models, comparing them and indicating in which situation is better to use an approach instead of another. Moreover, theoretical shortcut models and software simulations, not explicitly addressed by prior reviews, are taken into account. For researchers and designers this review provides a detailed methodology characterization as a guide to develop innovative study or project.
... In addition, the porous structure of biomass gives it anisotropic properties, with thermal conductivity across the grain direction being approximately one-third that of along the grain direction. In contrast, the diffusion to gas flow across the wood grain is much higher than that in the other two directions [27]. In a combustor/gasifier, a biomass particle undergoes a series of conversion processes, including initially drying and pyrolysis (devolatilization), subsequently partial oxidation of char and volatile, and finally, char combustion/gasification through reaction with an agent. ...
Biomass is an environmentally friendly renewable energy source and carbon-neutral fuel alternative. Direct combustion/gasification of biomass in the dense particle-fluid system is an important pathway to biomass energy utilization. To efficiently utilize biomass for energy conversion, a full understanding of biomass thermal conversion in lab/industrial-scale equipment is essential. This thesis aims to gain a deeper understanding of the physical and chemical mechanisms of biomass combustion/gasification in fluidized bed (FB) furnaces using computational fluid dynamics (CFD) simulations.
A three-dimensional reactive CFD model based on the Eulerian-Lagrangian method is developed to investigate the hydrodynamics, heat transfer, and gasification/combustion characteristics of biomass in multiple-scale FB furnaces. The CFD model considered here is based on the multi-phase particle-in-cell (MP-PIC)collision model and the coarse grain method (CGM). CGM is computationally efficient; however, it can cause numerical instability if the clustered parcels pass through small computational cells, resulting in the overloading of solid particles in the cells. To address this issue, a distribution kernel method (DKM) is proposed.
This method is to spread the solid volume and source terms of the parcel to the surrounding domain. The numerical stiffness problem caused by the CGM clustering can be remedied using DKM. Validation of the model is performed using experimental data from various lab-scale reactors. The validated model is employed
to investigate further the heat transfer and biomass combustion/gasification process.
Biomass pyrolysis produces a large variety of species in the products, which poses great challenges to the modeling of biomass gasification. A conventional single-step pyrolysis model is widely employed in FB simulations due to its low computational cost. However, the prediction of pyrolysis products of this model under varying operating temperatures needs to be improved. To address this issue, an empirical pyrolysis model based on element conservation law is developed. The empirical parameters are based on a number of experiments from the literature. The simulation results agree well with the experimental data under different operating conditions. The pyrolysis model improves the sensitivity of gasification product yields to operating temperature. Furthermore, the mixture characteristics of the biomass and sand particles and the effect of the operating conditions on the yields of gasification products are analyzed.
The validated CFD model is employed to investigate the fluidization, combustion, and emission processes in industrial-scale FB furnaces. A major challenge in the CFD simulation of industrial-scale FB furnaces is the enormous computational time and memory required to track quadrillions of particles in the systems. The
CFD model coupling MP-PIC and CGM greatly reduces the computational cost, and the DKM overcomes the unavoidable particle overloading issue due to the refined mesh in complex geometry. The CFD predictions agree well with onsite temperature experiments in the furnace. The CFD results are used to understand the granular flow and the impact of operating conditions on the physical and chemical processes in biomass FB-fired furnaces.
... During the pyrolysis, several reactions can take place simultaneously during the thermal decomposition of the feedstock [53]. Describing these reactions involves setting up models which cite the detailed reactions of the disintegration of the main components of the biomass as was done by the models of Ranzi [54] and Di Blasi [55,56]. Not being the main objective of this work, a single simplified reaction has been used to describe the biomass pyrolysis. ...
Valorization of flax shives biomass is investigated through pyrolysis and in-line catalytic hybrid reforming. We demonstrate that this approach enables the conversion of the biomass into three valuable products namely syngas, biochar, and bio-oil while reducing the emissions of greenhouse gases (CH4 and CO2). Given the increasing attention to syngas and especially hydrogen, pyrolysis and reforming reactor parameters were adjusted to enhance syngas production. Analyzed biochar properties such as contents of carbon (>82%), oxygen (<5%), and hydrogen (<1.5%) are conform to the European Biochar Certificate. The bio-oil composition analysis showed that the main compounds were benzene, toluene, phenols, guaiacol, syringol, furfural, and some polycyclic aromatic hydrocarbons, which were converted to lighter species by the reforming. Among the catalysts tested (cobalt, nickel, and cobalt-nickel supported on alumina beads), nickel provided the highest H2 production (8.26 mmol/gdry biomass or 37.41 vol.%). Nickel loading was varied from 0 to 25% with a maximum H2 production obtained with 20%Ni/Al2O3 (11.17 mmol/gdry biomass or 40.13 vol.%). Increasing the reforming temperature from 650 to 800°C enhanced the gas formation (53 wt.%) and the conversion rates of CH4 (62.6%) and CO2 (48.3%). An aging test was also performed on the 20%Ni/Al2O3 revealing a drop in the catalyst activity and stability. The catalysts characterization was realized by determining their porosity, specific surface area, weight loss, and SEM images. It revealed that sintering and carbon deposition are the main reasons behind their deactivation.
Graphical Abstract
... Because of its adhesive properties and aromaticity, it can be used as a "natural adhesive" to replace synthetic polymers derived from petroleum resources. Therefore, in the field of adhesive research, lignin can provide more environmentally friendly and sustainable solutions [21]. Unfortunately, the proportion of cellulose and lignin in contact with each other is reduced due to the dispersion of hemicellulose between them [22,23]. ...
An efficient way to alleviate the pollution imposed by petroleum-based supplies like synthetic fibres and plastics is to prepare biocomposites from recyclable forestry waste with a continuous supply. Despite this, it remains a significant challenge in the field of wood-based panel manufacturing to produce high-performance yet environmentally friendly wood-based materials without the addition of chemical adhesives. Lignin can be used as a “natural adhesive” due to its superior bonding properties, but the dispersion of hemicellulose affects cross-linking at the interfacial interface negatively. This study used lignin/cellulose as a matrix and pretreated it with hydrogen peroxide, sodium hydroxide, sodium silicate solution and in situ bonding of wood fibres to create a high-performance biocomposite material for potential mass production. The findings revealed the tensile (106.63 MPa) and bending strengths (148.78 MPa) of the optimised samples were 125.37% and 91.40% higher than the performance before optimisation. Moreover, the biocomposite demonstrated remarkable hydrophobicity, as evidenced by a water contact angle of 99.96°, and exhibited high thermal stability, without any disintegration observed even when subjected to combustion at 1300 °C. These exceptional properties and advantages render it a highly desirable material for eco-friendly homes and construction applications.
Graphical Abstract
... For the sake of model simplification, the volatiles are represented as a postulate substance C H O , whose x, y or z subscripts are calculated from the ultimate and proximate analysis. During devolatilization each biomass particle breaks down into the following 4 light gases [66][67][68] ...
This study presents CFD simulations of biomass dust explosions in a newly developed experimental 1 m$^3$ silo apparatus with variable venting, designed and fabricated to operate similarly to the explosivity test standards. The aim of the study is to validate a CFD model under development and investigate its capability to capture the transient effects of a vented explosion. The model is based on OpenFOAM and solves the multiphase (gas-particle) flow using an Eulerian-Lagrangian approach in a two-way regime. It considers the detailed thermochemical conversion of biomass, including moisture evaporation, devolatilization, and char oxidation, along with the homogeneous combustion of gases, turbulence, and radiative heat transfer. The explosion is analyzed in all stages, i.e., dust cloud dispersion, ignition, closed explosion, and vented explosion. The results indicate excellent agreement between the CFD model and experimental tests throughout the sequence. Our findings highlight the critical role of particle size in dust cloud distribution and pre-ignition turbulence, which significantly influences flame dynamics and the explosion itself. This model shows great promise and encourages its application for future investigations of biomass dust explosions in larger-scale geometries, especially in venting situations that fall out of the scope of the NFPA 68 or EN 14491 standards, and to help design effective safety measures to prevent such incidents.
... Some authors consider the dry wood as a lumped artificial specie, with its composition based on the proximate and ultimate analysis of the fuel. They model pyrolysis as a single step and first order reaction [29,32] or as the sum of three competing reactions with different reaction products (gas, tar and char) [41,42]. Other authors consider the decomposition of wood pseudo-components (cellulose, hemicellulose and lignin) with a kinetic rate for each component [43,44]. ...
... For the sake of model simplification, the volatiles are represented as a postulate substance C x H y O z , whose x, y or z subscripts are calculated from the ultimate and proximate analysis. During devolatilization each biomass particle breaks down into the following 4 light gases (Sami et al., 2001;Di Blasi, 2008;Neves et al., 2011 C x ...
... In the scale-up design of reactors, to reduce the computational costs of the computational fluid dynamics (CFD) simulations, the uniform conversion model is commonly utilized to deal with biomass particles without considering the intraparticle physical fields during thermal conversions [1,2]. By comparison, the meso-scale process receives less attention than the other two scale processes [3]. However, pyrolysis of large biomass particles at moderate temperatures has been demonstrated to occur in the thermally thick regime by estimating a pair of dimensionless quantities (Bi th ≫ 0.1, Py ≫ 10) [4]. ...
... The fuel conversion in fluidized bed gasification is, to a considerable extent, determined by the fuel pyrolysis ( [20,21]), in which the large solid fuel molecules are broken down by thermal cracking of chemical bonds. Pyrolysis products contain tar species, hydrocarbons (C x H y ) and CH 4 as well as impurities such as NH 3 , H 2 S, COS and HCl [22]. ...
The evaluation of lignocellulosic biomass applied to thermochemical routes is postulated as an alternative for the generation of energy from renewable sources. This work aims to compare biorefineries based on two thermochemical routes for the use of raw materials from the rice (husk and straw) production chain in the Department of Sucre-Colombia. Initially, this work analyzes the physicochemical and structural characterization of biomass. Four different scenarios are proposed for the comparison of the valorization from the simulation in Aspen Plus by means of fast pyrolysis and downdraft gasification. The novelty of this work is focused on the identification of the biorefinery with the best techno-economic, energetic, and environmental performance for the generation of electricity and value-added products from rice straw and husk. From an economic perspective, the stand-alone gasification process does not have a positive economic margin, which is an opposite behavior from the pyrolysis process. The biorefinery proposed in scenario 1 (fast pyrolysis of both rice residues) had the best economic and environmental performance with an economic margin of 13.75% and emissions of 2170.92 kgCO 2 eq/kg for 10 years. However, this scenario was not energetically the best, holding second place due to the feedstock requirements, compared to gasification. The biorefinery scenario 1 has the best performance.
En la actualidad hay una gran producción energética realizada por diferentes métodos de transformación siendo elpetróleo y sus derivados la mejor forma de obtención de energia, sin embargo, las energías renovables se están posicionando como una fuente limpia, económica y eficiente de obtener energía, entre estas destacan la energía eólica, fotovoltaica y de la biomasa, siendo esta ultima el objeto de estudio del siguiente trabajo que se ha realizado con el fin de llevar a cabo diversos análisis, para determinar el potencial energético que contienen diferentes residuos de cosechas típicas del estado de Chiapas, las cuales no son tratadas y generan problemas ambientales desaprovechando su recurso energético almacenado en sus fibras, siendo el olote de maíz, cáscara de coco, cáscara de cacahuate y cascabillo de maíz; los residuos a analizar, tomando una muestra de 500 g de cada uno de los residuos, para realizar todos los análisis, como lo son: análisis elemental, termogravimétrico y poder calorifico, además de realizar un análisis matemático para conocer el porcentaje de celulosa, hemicelulosa y lignina a partir de los datos obtenido mediante el análisis fundamental. la muestra de 500 g se pulverizo hasta llegar a un tamaño de particula inferior a los 2 mm. Las caracterizaciones realizadas fueron análisis inmediato, analisis elemental, análisis termogravimétrico, análisis de composición de biomasa en base seca calculada por balance atómico y determinación del poder calorífico mediante una bomba isoperibólica.
The potential of spent coffee grounds (SCG) to act as a replacement fuel material for the malt-drying process during whisky production was evaluated. The extracts of both materials, and the smoke they produced through burning, were subjected to analysis by high resolution ¹H NMR spectroscopy. Malts infused with the smokes were similarly studied to gain an understanding of the transfer of chemical species from smoke to grain. In addition, the thermal degradation of peat and SCG were investigated using thermogravimetric analysis and pyrolysis – GCMS. Our studies revealed that, despite some chemical differences between the source materials, the composition of the smoke produced by both is remarkably similar. It may be concluded that the aroma and flavour of the spirit, resulting from substitution of peat by SCG is also likely to be similar, however the presence of additional congeners in the SCG-derived spirit, including furans and methylpyridines, could introduce undesirable off notes.
Charcoal is an important source of renewable biomass and has great industrial importance as a biocarbon in the production of pig iron and steel. To increase the quality and yield of charcoal, it is necessary to invest in the continuous improvement of kilns. However, the development process for slow carbonization kilns often lacks structure and formalization, relying heavily on experimentation. This absence of structured guidance for kiln design necessitates research in this domain. This study addresses this research gap by consolidating both tacit and formal knowledge concerning carbonization kilns. Based on this, it identifies and discusses aspects for the development and improvement of slow carbonization kilns for charcoal production. The study also elucidates the pivotal variables in the charcoal production process and their interplay with the overarching characteristics of carbonization kilns. Furthermore, it employs functional modeling to establish solution principles for each of the kiln's secondary functions. The results of this study have the potential to stimulate the development and enhancement of charcoal production technologies, leading to increased productivity, yield, and charcoal quality, in addition to the reuse of gases.
In this work, the influence of gas–solid drag and heat transfer coefficient models on the prediction capacity of the Euler–Euler approach to simulate reactive bubbling fluidized bed flows is studied. Three different cases are considered, a non-reactive bidisperse bubbling fluidized bed flow (Case 1), and two reactive polydisperse flows in bubbling fluidized beds, one for biomass gasification (Case 2), and the other for biomass pyrolysis (Case 3). The Gidaspow, Syamlal–O’Brien, and BVK gas–solid drag models and the Gunn, Ranz–Marshall, and Li–Mason gas–solid heat transfer correlations are investigated. A Eulerian multiphase approach in a two-dimensional Cartesian domain is employed for the simulations. Computational results for the three cases are compared with experimental data from the literature. The results obtained here contribute to a better understanding of the impacts of such closure models on the prediction ability of the Euler–Euler approach to simulate reactive flows. The results indicate that, for the simulation of reactive flows in bubbling fluidized bed reactors, the kinetic modeling of the reactions has a global effect, which superposes with the influence of the drag and heat transfer coefficient models. Nevertheless, local parameters can be noticeably affected by the choice of the interface closure models. Finally, this work also identifies the models that lead to the best results for the cases analyzed here, and thus proposes the use of such selected models for gasification and pyrolysis processes occurring in bubbling fluidized bed reactors
Lignin, the second most abundant biopolymer on earth and with a predominantly aromatic structure, has the potential to be a raw material for valuable chemicals and other bio‐based chemicals. In industry, lignin is underutilized by being used mostly as a fuel for producing thermal energy. Valorization of lignin requires knowledge of the structure and different linkages in the isolated lignin, making the study of structure of lignin important. In this article, lignin samples isolated from two types of reactors (autoclave reactor and displacement reactor) were analyzed by FT‐IR, size exclusion chromatography, thermogravimetric analysis (TGA), and Py‐GC‐MS. The average molecular mass of the organosolv lignins isolated from the autoclave reactor decreased at higher severities, and FT‐IR showed an increase in free phenolic content with increasing severity. Except for molecular mass and molecular mass dispersity, there were only minor differences between lignins isolated from the autoclave reactor and lignins isolated from the displacement reactor. Carbohydrate analysis, Py‐GC–MS and TGA showed that the lignin isolated using either of the reactor systems is of high purity, suggesting that organosolv lignin is a good candidate for valorization.
3D biomass‐derived carbon (3D‐BC) materials are widely developed for various electrochemical sensors due to their structural diversity and high electrical conductivity, and are gradually experiencing their most prosperous moment. A timely and comprehensive review of structure‐strategy‐property will greatly broaden this research field. In this paper, the mechanism of biomass‐derived carbon conversion is presented and the recent progress of 3D‐BC materials for constructing high‐performance electrochemical sensors, including biomass sources, carbonization methods, and synthesis strategies for 3D‐BC is summarized, as well as their applications in different electrochemical sensors are reviewed. Finally, the challenges and opportunities for the future development of biomass‐derived carbon (BC) materials are emphasized. In conclusion, this review will assist researchers in selecting suitable precursors and strategies to design BC materials and facilitate their application in future electrochemical sensors.
The combined discrete element method/computational fluid dynamics (DEM/CFD) approach allows for the description of reacting granular assemblies passed by a gas flow. Especially for thermally thick particles, the spatial resolution of the solid object volumes, their surfaces, and of the interstitial flow domain controls the quality of results while at the same time driving computational costs. To evaluate the differences between resolved and unresolved DEM/CFD approaches, pyrolysis of a bulk of spherical wood particles enclosed by a cylindrical heating surface and passed by hot nitrogen is numerically examined. The unresolved simulation is based on the averaged volume method (AVM). For the resolved simulation, the so‐called blocked‐off (BO) method is applied. The results show that the total mass conversion rate of solid particles into gaseous volatiles is faster when employing the resolved BO approach. The better spatial resolution of local flow field and particle surface representation leads to a more detailed prediction of convective and radiative heat transfer to the particles, but is associated with the penalty of a six‐fold computing time.
This study's objective was to emphasize the renewable lignocellulosic biomasses' energy potential by thermally describing them. The TG/DTG/DTA procedures were used to examine the thermal pyrolysis behavior of coffee wastes (CW), date seeds (DSM), prickly pears (PP), and two blends (50%CW/50%DSM and 33.3%CW/33.3%DSM/33.3%PP by weight), and their physico-chemical compositions were identified and assessed. The activation energy (E) of all tested samples was predicted using iso-conversional models like Friedman (FR), Starink (ST), Kissinger-Akahira-Sunose (KAS), and Ozawa-Flynn-Wall (FOW), and the frequency factor (A) was calculated based on the acquired (E) values utilizing the chemical reaction mechanisms (C1-C8). SEM, EDX, and FTIR techniques were used to examine the microstructure, mineralogy, and functional group present in the studied samples. Every DTG profile displayed a rightward shift when the heating rate (β) rose to higher temperatures. The difference in the average activation energy (Eav) values between the ST, OFW, and KAS models, except for the blend (1), does not surpass 5 kJ⁄mole. Increases in the (β) value, particularly for blends, were significantly correlated with increases in the maximum pyrolysis rate (−RP), pyrolysis index (CPI), pyrolysis stability index (RW), and volatile release index (Ddev). The ΔGav discrepancies between the results from all kinetic models for all samples varied from 1 to 7 kJ⁄mole. A little
potential energy barrier was noticed when comparing the (Eav) to the average enthalpy (ΔHav).
This study's objective was to emphasize the renewable lignocellulosic biomasses' energy potential by thermally describing them. The TG/DTG/DTA procedures were used to examine the thermal pyrolysis behavior of coffee wastes (CW), date seeds (DSM), prickly pears (PP), and two blends (50%CW/50%DSM and 33.3%CW/33.3%DSM/33.3%PP by weight), and their physico-chemical compositions were identified and assessed. The activation energy (E) of all tested samples was predicted using iso-conversional models like Friedman (FR), Starink (ST), Kissinger-Akahira-Sunose (KAS), and Ozawa-Flynn-Wall (FOW), and the frequency factor (A) was calculated based on the acquired (E) values utilizing the chemical reaction mechanisms (C1-C8). SEM, EDX, and FTIR techniques were used to examine the microstructure, mineralogy, and functional group present in the studied samples. Every DTG profile displayed a rightward shift when the heating rate (β) rose to higher temperatures. The difference in the average activation energy (Eav) values between the ST, OFW, and KAS models, except for the blend (1), does not surpass 5 kJ⁄mole. Increases in the (β) value, particularly for blends, were significantly correlated with increases in the maximum pyrolysis rate (−RP), pyrolysis index (CPI), pyrolysis stability index (RW), and volatile release index (Ddev). The ΔGav discrepancies between the results from all kinetic models for all samples varied from 1 to 7 kJ⁄mole. A little
potential energy barrier was noticed when comparing the (Eav) to the average enthalpy (ΔHav).
A 3 kg/h ablative pyrolysis reactor and a 1.5 kg/h fluid bed reactor were operated using pine wood as the feedstock over a temperature range of 450–600°C. Mass balances and product analyses were performed and product comparisons made. A similar gas/vapour product residence time of around 1 s was used to reduce differences. Product yields followed the same general trends with a maximum organics liquid yield around 500–515°C for both reactors (59.4 wt% organics at 515°C and 1.19 s residence time in the fluid bed reactor and 62.1 wt% organics at 502°C and 1.1 s residence time for the ablative reactor]. Char yields increased for the fluid bed reactor above 515°C, and also in the ablative pyrolysis reactor. The volatile content of the char products were higher for the ablative, compared to the results for the fluid bed chars which showed a continual rapid decrease. Water yields were similar in the range of 11–16 wt%. Gas yields were markedly lower in the ablative pyrolysis reactor suggesting a less severe environment for the vapour products. The gaseous product composition variation with temperature in the fluid bed suggests that secondary vapour cracking is more prevalent in the fluid bed than the ablative pyrolyser.
A co-current moving bed gasifier with internal recycle and separate combustion of pyrolysis gas has been developed with the aim of producing a design suitable for scaling-up downdraft gasifiers while maintaining a low tar content in the producer gas. Using wood chips with a moisture content of 7–9 wt% (db) as a fuel at a rate of 20 kg h−1, this system produced a gas with a heating value of 4500 kJ ms−3 and a very low tar content of < 0.1 gms−3.
A detailed mathematical model is presented for the temporal and spatial accurate modeling of solid-fluid reactions in porous particles for which volumetric reaction rate data is known a priori and both the porosity and the permeability of the particle are large enough to allow for continuous gas phase flow. The methodology is applied to the pyrolysis of spherically symmetric biomass particles by considering previously published kinetics schemes for both cellulose and wood. A parametric study is performed in order !o illustrate the effects of reactor temperature, heating rate, porosity, initial particle size and initial temperature on char yields and conversion times. It is observed that while high temperatures and fast heating rates minimize the production of char in both reactions, practical limits exist due to endothermic reactions, heat capacity and thermal diffusion. Three pyrolysis regimes are identified: 1) initial heating, 2) primary reaction at the effective pyrolysis temperature and 3) final heating. The relative durations of each regime are independent of the reactor temperature and are approximately 20%, 60% and 20% of the total conversion time, respectively. The results show that models which neglect the thermal and species boundary layers exterior to the particle will generally over predict both the pyrolysis rates and experimentally obtainable tar yields. An evaluation of the simulation results through comparisons with experimental data indicates that the wood pyrolysis kinetics is not accurate; particularly at high reactor temperatures.
The working hypothesis for the study was that the main part of the chlorine in biomass is in an inorganic form and therefore should not vaporize appreciably below the melting point of the corresponding salt (around 700 °C) because the vapor pressure over solid salt is negligible. In the study, biomass fuels (sugarcane trash, switch grass, lucerne, straw rape) were subjected to pyrolysis in a flow of nitrogen, and the weight of the residue and its chlorine content were measured and compared to the original fuel. Contrary to the hypothesis, the results showed that during pyrolysis of biomass 20−50% of the total chlorine evaporated already at 400 °C, although the majority of the chlorine was water soluble (in grass 93%) and therefore most probably ionic species. At 900 °C, 30−60% of the chlorine was still left in the char. At 200 °C less than 10% of chlorine had evaporated from the fuel, indicating that the chlorine is not associated with water. Another result was that there was no significant difference in the chlorine release between biomass and synthetic waste, i.e., a mixture of organic and inorganic chlorides. These results are contradictory with the starting hypothesis and can therefore have new implications for the use of these fuels in combustion and gasification processes.
Sugar cane bagasse, wheat straw, pine and cotton wood pyrolysis was studied by TGA in argon at heating rates of 5 and 20°C/min. The DTG (-dm/dt) peaks associated with the components of an untreated plant material are relatively wide and strongly overlap each other. A reduction in the amount of inorganic ions in the samples by simple water or dilute acid washing procedures resulted in sharper peaks with a better separation. The thermal decomposition of the major biomass components was described, more or less formally, by first order reactions and the DTG curves of the biomass samples were approximated by a linear combination of first order reactions. Good fits between the calculated and the experimental data and good reproducibility of the model parameters were achieved. The kinetic model applied here may serve as a starting point to build more complex models capable of describing the thermal behavior of plant materials during a thermal or thermochemical processing or burning. Theoretically, there is also a possibility to utilise this sort of calculation in the quantitative analysis of the cellulose and hemicellulose content of the lignocellulosic materials.
The pyrolysis of biomass is a thermal treatment which results in the production of char, liquid and gaseous products.
The aim of this paper is the study of the influence of kinetic and diffusion phenomena on the pyrolysis of biomass particles.
In our aboratory the pyrolysis process has been studied experimentally using thermogravimetric techniques and different scales of apparatus.
Experimental data suggest that the pyrolysis of fine particles can be controlled by kinetics. The rate of pyrolysis of biomass can be well represented by the sum of the corresponding rates for the main biomass components.
The effect of particle size has also been studied by measuring the weight-loss rate. For particles below 1 mm in diameter the process is controlled by kinetics, for larger particles the process is controlled by both heat transfer and primary and secondary pyrolysis reactions. As the temperature and the particle size increase the relative influence of transfer phenomena and secondary reactions increases.
The temperature profiles inside the particles during pyrolysis were also measured.
Upon heating, biomass materials undergo solid phase pyrolysis at relatively low temperatures (> 300 ºC), forming reactive volatile matter, a few permanent gas species and solid char. Unlike the various coals and peats, biomass materials typically lose 70% or more of their weight by the solid phase pyrolysis reactions. This transformation of the bulk of the biomass material from the solid to the vapor phase suggests the important role of vapor phase chemistry in the pyrolysis of biomass materials. Recognizing the highly reactive nature of the major constituents of the volatile matter, the significance of the vapor phase chemistry becomes even more apparent.
Recent studies and information on principal thermal reactions and products of cellulosic materials, and some of the proposed thermal conversion processes are reviewed to indicate the future possibilities and promises of this approach.
Many types of chemical pathways aimed at representing the primary process of biomass degradation have been published. These models include or ignore the possibility of the existence of an intermediate “Active” species or state. The purpose of this paper is to gather theoretical results and experimental observations intended to open the discussion on the possible existence and nature of such an intermediate.
The results of the modelling of the chemical and thermal behaviour of biomass undergoing a pyrolysis decomposition are first given. Simple experimental and visual observations associated with theoretical considerations based on heat transfer measurements lead then to the conclusion that the overall reaction is similar to a fusion with production of an intermediate liquid species. From kinetic rate constants derived from literature, it appears that it is not necessary to take into account such a liquid in thermogravimetric analysis (TGA) experiments, but that it cannot be ignored in high temperature ablative pyrolysis conditions.
At the end of the paper, a discussion is conducted on the possible chemical natures of these liquids, the evolved vapours and the condensed pyrolysis oils formed in a pyrolysis process. It is concluded that the primary liquids could be composed of dimers and higher oligomers derived from cellulose and lignin, while the vapours would be mainly monomers and monomer fragments from the cracking of oligomers in the liquid phase.
A transient, one-dimensional model which can simulate drying and pyrolysis of moist wood is presented. The porous wood is divided into four phases: solid, bound water, liquid water and gas phases. Conservation equations for energy and mass together with Darcy’s law for velocity and algebraic equations for the transport properties and physical properties are presented. The drying model is based on equilibrium between water vapor and bound or liquid water in the porous wood. For the thermal degradation process, two reaction schemes including a single one-step global and a multiple competitive reaction model which have been proposed in the literature, are included. A comparison between the two pyrolysis models has been made on dry wood and the results reveal that for the multiple reaction model, the heating rate and pyrolysis time have great influence on the ultimate char, tar and gas yields calculated. Simultaneously drying and pyrolysis of large wood particles are simulated. The effect of moisture content on the pyrolysis time is presented together with characteristic profiles for temperature, pressure and moisture distribution inside the particle. A temperature plateau can be observed at about 100°C where evaporation and condensation of liquid water takes place. The simulations show that the in-depth moisture content increases and exceeds the initial moisture content before evaporation. The reason for this increase is that when water evaporates in the front, some of it will be transported by convection and diffusion into the colder region and condensed.
A normative review of the literature describing the products, mechanisms, and rates of lignin and whole biomass pyrolysis is presented. The role of a complex sequence of competing solid- and vapor-phase pyrolysis pathways is elucidated.
The liquid products (tars or sirups) obtained from the fast pyrolysis of cellulosics show a wide variation in composition depending on the cellulosic feedstock used. Native cellulose in wood gives significant yields of hydroxyacetaldehyde and other low molecular weight oxygenated compounds but low yields of anhydrosugars, while highly altered micro-crystalline cellulose gives the reverse. Experimental results from fluidzed bed fast pyrolysis are given for poplar wood and for a number of types of cellulose produced by different processes. The effect on product nature and yields as a result of different pretreatments of the wood or cellulose before pyrolysis is also reported. From these observations, as well as from the variation of product yields with temperature for one cellulose product, possible mechanisms for the primary decomposition of cellulose are proposed. Two major parallel pathways appear to account for the yields of major products. The content and nature of inorganic salts and the degree of polymerization of the cellulose play an important role in determining the relative importance of these two decomposition pathways.
A pyrolysis kinetics model is presented which can be used for the prediction of pyrolysis oil yields for oak in an entrained-flow reactor. The parameters in the model were determined by a nonlinear least-squares computer code and experimental results. An interpretation of the model predictions is included. The maximum oil yield obtained experimentally was 51%. The model predicts that a maximum oil yield of 63% is possible at a temperature of 550 degree C (the highest temperature run to date) at a much higher inlet gas rate than has been used.
When wood is heated at elevated temperatures, it will show a permanent loss of strength resulting from chemical changes in its components. The thermal decomposition can start at temperatures below 100°C if wood is heated for an extended period of time. Figure 1 shows that wood heated at 120° loses 10% of its strength in about one month, but i t takes only one week to obtain the same loss of strength if it is heated at 140° (1). Heating at higher temperatures gives volatile decomposition products and a charred residue. The pyrolytic reactions and products control the combustion process and relate to the problems of cellulosic fires, chemical conversion of cellulosic wastes and utilization of wood residues as an alternative energy source. In our laboratory, the pyrolytic reactions of wood and its major components have been investigated by a variety of analytical methods. Thermal analysis of cottonwood and its
A heat radiation reactor was used to study the mechanism of cellulose rapid pyrolysis in the present paper. Combined with gas chromatography-mass spectrometry analysis on bio-oil, experimental results showed that the production rate of hydroxyacetaldehyde and 1-hydroxy-2-propanone, as well as their proportions in bio-oil, increased with the reaction temperature in the case of short gas residence time. The formation of the above two products was shown to be competitive with levoglucosan formation from active cellulose. In addition, a modified cellulose pyrolysis model based on the Brodio-Shafizadeh model was proposed to describe this competitive phenomenon. Major products' formation was simulated with this modified model, which was in good agreement with the experimental results.
The influence from structural changes, heat transfer properties of dry wood and pyrolysis mechanism on the pyrolysis of large wood particles were studied. Measurements of temperature distribution and mass loss were performed on cylindrical samples of dry birch wood during pyrolysis in an inert atmosphere at 700°C. A model of wood pyrolysis was modified to include structural changes. Comparisons of measurements and model simulations show that the inclusion of a shrinking model reduces the time of pyrolysis substantially. The varying interior heating rate was found to influence the choice of pyrolysis mechanism. Of four pyrolysis mechanisms found in literature, only one was found to be in agreement with the measurements.
This paper examines the effects of intraparticle heat and mass transfer on the devolatilization of millimeter-sized biomass particles under conditions similar to those found in commercial coal-fired boilers. A computational model is presented that accounts for intraparticle heat and mass transfer by diffusion and advection during particle heating, drying, and devolatilization. To evaluate the model, devolatilization experiments under high-temperature and high-heating rate conditions were conducted using the Multifuel Combustor at Sandia National Laboratories. Measurements of mass-loss and changes in particle size for millimeter-sized alfalfa and wood particles are presented as a function of reactor residence time. For millimeter-sized particles, both fuels completely devolatilized in approximately 1 s with rapid initial mass loss. The total volatile yield of the wood was 92% on a dry, ash-free basis, significantly higher than that reported by a standard ASTM test, indicating dependence of the ultimate yield on local conditions. Particles for both fuels shrink significantly and become less dense during devolatilization. The comprehensive model accurately predicts the devolatilization behavior of millimeter-sized biomass particles; these measurements could not be reproduced with a simple lumped model that ignores intraparticle transport effects. The comprehensive model is used to examine the effects of particle size and moisture content on devolatilization under conditions representative of those found in coal boilers. Biomass particles of radii up to 2 mm and moisture content up to 50% are considered. As expected, intraparticle heat and mass effects are more significant for larger particles. These effects can significantly delay particle heating and devolatilization; for example, intraparticle effects delay the heating and devolatilization of millimeter-size particles by as much as several seconds for a particle with a 1.5-mm radius compared to predictions of a lumped model. This delay is significant considering the short residence times of commercial boilers and should be accounted for in computational models used to evaluate the effects of biomass-coal cofiring on boiler performance.
We present here a decoupling technique to tackle the entanglement of the nonlinear boundary condition and the movement of the char/virgin front for a thermal pyrolysis model for charring materials. Standard numerical techniques to solve moving front problems — often referred to as Stefan problems — encounter difficulties when dealing with nonlinear boundaries. While special integral methods have been developed to solve this problem, they suffer from several limitations which the technique described here overcomes. The newly developed technique is compared with the exact analytical solutions for some simple ideal situations which demonstrate that the numerical method is capable of producing accurate numerical solutions. The pyrolysis model is also used to simulate the mass loss process from a white pine sample exposed to a constant radiative flux in a nitrogen atmosphere. Comparison with experimental results demonstrates that the predictions of mass loss rates and temperature profile within the solid material are in good agreement with the experiment. Copyright © 1999 John Wiley & Sons, Ltd.
Biomass pyrolysis studies were conducted using both a thermogravimetric analyser and a packed-bed pyrolyser. Each kind of biomass has a characteristic pyrolysis behaviour which is explained based on its individual component characteristics. Studies on isolated biomass components as well as synthetic biomass show that the interactions among the components are not of as much significance as the composition of the biomass. Direct summative correlations based on biomass component pyrolysis adequately explain both the pyrolysis characteristics and product distribution of biomass. It is inferred that there is no detectable interaction among the components during pyrolysis in either the thermogravimetric analyser or the packed-bed pyrolyser. However, ash present in biomass seems to have a strong influence on both the pyrolysis characteristics and the product distribution.
The effects of cations on the yields of char, tar, light oils, and total gases from rapid pyrolysis of beech wood were studied. Raw wood, acid washed wood, and wood impregnated with potassium, sodium, and calcium cations were pyrolyzed in 1 atm pressure of helium at 1000°C s−1 heating rate to a peak temperature of ≈ 1000°C. Experiments were carried out in an electrical screen heater reactor, and the yields of products were determined as a function of pyrolysis peak temperature. Acid washed wood samples gave the highest tar yield (about 61% by weight of the original wood), whereas wood samples impregnated with potassium or sodium cations gave the lowest yield of tar (32 wt%). The char yield from acid washed wood samples was lower (about 6 wt%), and it was higher for raw wood and for wood samples impregnated with different cations (10–15%). The maximum gas yield was lower at ≈ 34 wt% for acid washed wood and much higher (≈ 58 wt%) for the wood samples impregnated with potassium and sodium cations. These results, as expected, confirm the marked catalytic effects of cations on the post-pyrolysis cracking reactions of tar (the major product of wood pyrolysis) and the formation of char and gaseous products via cracking reactions of tar. In all these cases, sodium and potassium cations showed a stronger effect than calcium cations. The tar molecular weight was measured for wood samples impregnated with different cations. Tar molecular weight decreased with addition of cations to the wood particles, whereas it remained relatively constant with pyrolysis temperature. The tar molecular weight dropped from about Mw = 300 amu and Mn = 155 amu for raw and acid washed woods to about Mw = 190 amu and Mn = 100 amu for woods impregnated with potassium or sodium cations.
A Setaram DSC in conjunction with stainless steel pressure vessels was used to investigate the effects of pressure and purge gas flow rate (gas phase residence time) on the heat demands of cellulose pyrolysis. High pressure and low flow rate reduce the heat of pyrolysis and increase char formation. Experiments were conducted to investigate the pyrolysis reactions of anhydrocellulose and levoglucosan, the two major intermediate products in cellulose pyrolysis. Separate models for the degradation of each intermediate were postulated and combined to form a detailed mechanistic model for cellulose pyrolysis. The model explains all the observed effects of pressure and flow rate.
Rice hulls were pyrolyzed in a thermogravimetric analyzer in a helium atmosphere to determine the kinetic parameters of devolatilization reactions. The pyrolysis experiments were conducted by heating rice hulls from room temperature to 1173 K at constant heating rates of 3, 10, 30, 60, and 100 K/min. The global mass loss during rice hull pyrolysis was successfully simulated by a combination of four independent parallel reactions, the decompositions of four major components in rice hulls: moisture, hemicellulose, cellulose, and lignin. The activation energy for the decomposition of the nonmoisture components was in the order cellulose > hemicellulose > lignin. It was also found in the present study that the pyrolytic behaviors were significantly influenced by water wash prior to pyrolysis. The water wash elevates the peak temperature and the activation energy for the decomposition of each component of rice hulls. The volatile yields resulting from cellulose and hemicellulose decompositions during rice hull pyrolysis increase due to the water treatment, whereas those from lignin decomposition and the char yield decrease.
The homogeneous vapor phase cracking of newly formed wood pyrolysis tar was studied at low molar concentrations as a function of temperature (773 - 1.073 K), at residence times of 0.9 - 2.2 s. Tar conversions ranged from about 5 to 88%. The tars were generated by low heating rate (0.2 K/s) pyrolysis of --2 cm deep beds of sweet gum hardwood, and then rapidly conveyed to an adjacent reactor for controlled thermal treatment. Quantitative yields and kinetics were obtained for tar cracking and resulting product formation. The major tar conversion product was carbon monoxide, which accounted for over two-thirds of the tar lost at high severities. Corresponding ethylene and methane yields were each about 10% of the converted tar. Coke formation was negligible and weight-average tar molecular weight declined with increasing tar conversion.
Improved experimental techniques are described, using a wire mesh reactor; for determining the pyrolysis yields of lignocellulosic materials. In this apparatus pyrolysis tars are rapidly swept from the hot zone of the reactor and quenched, secondary reactions are thereby greatly diminished. Particular emphasis is placed upon the measurement of the pyrolysis yields for sugar cane bagasse, an abundant agricultural waste product. The role of the important pyrolysis parameters, peak temperature and heating rate, in defining the ultimate tar yield is investigated, with the value for bagasse being 54.6% at 500 C and 1,000 C/s. The pyrolysis yields, under similar conditions, of another biomass material, silver birch, are also reported and compared to those of bagasse.
In recent years there has been a growing interest in using wood, bark and foliage for chemical biomass conversion studies.1 The reasons are understandable since forests represent one of the largest sources of renewable biomass still available to mankind. Also, the Forest Product Industries provide a constant, collected source of potential thermochemical conversion material such as tops, limbs, bark and foliage not required for lumber or pulp. This potential will increase dramatically if plans to introduce whole tree logging materialize. Unfortunately, many scientists have been attracted to this potential biomass who are unfamiliar with the wide variation among and between tree species. To many wood is wood and they make little attempt to define the sample on which valuable scientific research is done. Borrowing a sentence from the Basic Coal Sciences Project Advisory Report2 and substituting wood for coal, the following statement emerges and describes the current situation concisely: ‘Considerable basic research has been done on a wide range of wood samples for various purposes, yet much of this previous research cannot be correlated since little, if any comparisons can be drawn from the samples used’. The Estes Park conference unanimously endorsed the need for reviewers and authors to be aware of tree variability and to define all research samples in such a way that other scientists may be able to calibrate and correlate their own research results.
Seeking to systemalically identify and organize the physical and chemical properties of biomass particles for combustion in furnaces, an attempt is made here to theoretically describe the temporal change in mass of a burning particle of an organic solid. The development involves a series of simple models to highlight the mechanisms involved, the physical and chemical properties of concern and the expected results. Although many simplifying assumptions are made, the properties identified, the parameters which evolved and the final functional form of the results are expected to becorrect.
A mathematical model based on unreacted-core shrinking model is developed to describe the pyrolysis phenomena of the cellulosic materials in the temperature range of 700–1470 K. The relative importance of each model parameter is studied by applying the sensitivity analysis. The heat transfer control region and kinetic reaction control region are obtained from the developed model. The shift of controlling mechanism is dependent on the particle size and the wall temperature. The effect of heat of reaction on the temperature of the particle is also discussed.
Single, thermally thick particles of lodgepole pinewood were pyrolyzed under well-defined conditions of industrial importance. Particle thickness, heating level, moisture content, density, and grain axis relative to one-dimensional heating were varied using a Box-Behnken experimental design. Gross product fractions, as well as components therein, were measured and the batch yields were correlated with second-order polynominals. The empirical equations correlating the batch yields, together with their prediction uncertainties, are presented and are suitable for use in simulations of wood combustion and thermal conversion. Comparison of large particle pyrolysis product distributions to other studies of small-particle pyrolysis yields shows the trends with particle size to be consistent. Tar yield minima depend on both particle size and heating rate. Gas yield is dependent on both particle size and heating intensity. Because some process controllables were found to alter product yields from large particles in a multiplicative way, rather than an additive way, suggestions for future experiments are made.
Thermogravimetric data on the devolatilization rate of beech wood are re-examined with the aim of incorporating the effects of high heating rates (up to 108Kmin−1) in the global kinetics. The mechanism consisting of three independent parallel reactions, first-order in the amount of volatiles released from pseudo-components with chief contributions from hemicellulose, cellulose and lignin, is considered first. It is found that the set of activation energies estimated by Gronli et al. [M.G. Gronli, G. Varhegyi, C. Di Blasi, Ind. Eng. Chem. Res. 41 (2002) 4201–4208] (100, 236 and 46kJmol−1, respectively) for one slow heating rate results in very high deviations between predicted and measured rate curves. The agreement is significantly improved by a new set of data consisting of activation energies of 147, 193 and 181kJmol−1, respectively. In this case, the overlap is reduced between the reaction rates of the three pseudo-components whose chemical composition is also modified. In particular, instead of a slow decomposition rate over a broad range of temperatures, the activity of the third reaction is mainly explicated along the high-temperature (tail) region of the weight loss curves. The performances of more simplified mechanisms are also evaluated. One-step mechanisms, using literature values for the kinetic constants, produce large errors on either the conversion time (activation energy of 103kJmol−1) or the maximum devolatilization rate (activation energy of 149kJmol−1). On the other hand, these parameters are well predicted by two parallel reactions, with activation energies of 147 and 149kJmol−1.
A comparison between the thermal decomposition of almond shells and their components (holocellulose and lignin) was carried out, considering the yields of the most important products, under flash conditions, and the decomposition kinetics.The yields of the main gaseous products obtained in the fast pyrolysis of almond shells can be reproduced from the yields obtained with holocellulose and lignin. The best results were obtained with CO, water and CO2. The differences were greater with the minor hydrocarbons, CH4, C2H6, C2H4, etc.The kinetics of the slow thermal decomposition (TG-DTG) of almond shells cannot be reproduced by the sum of lignin and holocellulose. The cellulose from almond shells decomposes at lower temperatures than almond shells, and the behavior of isolated lignin is very different from that found when it forms part of the raw material, proving the importance of the interactions between its components.
Drying and devolatilization are studied at combustion temperatures. The surface temperature of particles at the end of drying can significantly exceed the temperature when devolatilization starts, implying that drying and pyrolysis may partly overlap. Devolatilization is controlled by heat transfer, when the particle size is large. The critical particle size at which heat transfer dominates chemical kinetics is discussed. A model for calculating the intrinsic rate of generation of volatiles in the regime of heat transfer control is presented. A novel isotherm migration method is used for the computation of simultaneous drying and pyrolysis inside a fuel particle. It applies to the study of heat transfer in a one-dimensional geometry with moving phase-change boundaries, internal fluid flow and mass generation, including steep temperature and density profiles, as frequently encountered in combustion.
In this paper, the weight loss of four different woods during vacuum pyrolysis at constant temperature and at constant heating rate are reported. Based on these results, an empirical model of the weight loss rate as a function of temperature has been developed. The model assumes that wood is composed of three major components: cellulose, lignin, and a mixture of hemicellulose and volatiles. Under vacuum conditions, it is also assumed that the pyrolysis of these components proceeds independently of the others. The resulting kinetic parameters can be used to predict volatilization rates of wood as a function of temperature in a vacuum. The model may also be used to estimate the quantities of each of the main components initially present in an unknown wood sample.
A lumped-parameter kinetic model is applied to simulate the pyrolysis of lignocellulosic particles, exposed to a high temperature environment. Physical processes account for radiative, conductive and convective heat transport, diffusion and convection of volatile species and pressure and velocity variations across a two-dimensional (2-D) , anisotropic, variable property medium. The dynamics of particle degradation are found to be strongly affected by the grain structure of the solid. A comparison is made between the total heat transferred to the virgin solid (conduction minus convection) along and across the grain. Notwithstanding the lower thermal conductivities, because of the concomitant slower convective transport (lower gas permeabilities) , the largest contribution is that across the solid grain. The role played by convective heat transport is successively less important as the particle size is increased. Finally, the 2-D and the widely applied one-dimensional (1-D) predictions are compared.
Pyrolysis experiments in a thermogravimetric analyser and in a muffle furnace were carried out with spruce and beech wood. The particle size of the samples of spruce wood was varied in the range 0.5–20 mm and the heating rate was varied in the range 5–60 K/min. Additionally, drop-in experiments in the muffle furnace were carried out with both spruce and beech wood. These experimental data were compared with the results from calculation of pyrolysis of ‘large’ particles with the simulation program parsim. Agreement could only be obtained when the tar decomposition outside of the particle was taken into account. The pre-exponential factor and the ultimate yield for cracking of tar from beech wood are given.
This work deals with the kinetic analysis of data obtained in thermobalance. It consists of a review of kinetic models used for material decomposition, and also of the methods used for the analysis. The topics presented comprise numerical problems related with kinetic analysis, best conditions for a kinetic run, discussion about correlation vs. actual models, nth order models, more complex models (models using a great number of reactions, models using various fractions), calibration of the temperature, position of the thermocouple, sample mass and particle size. Other topics treated are the validation of kinetic models using MS data and the different models available for kinetic studies in thermobalance.
Release of volatiles of non-spherical pine wood particles was analysed by means of continuous measurements of the CO2 and O2 concentrations obtained after the complete combustion of the volatiles and from flame extinction times. The effect of the atmosphere used for devolatilisation was tested. The volatiles' evolution was nearly identical using air or N2 as fluidising gas. The devolatilisation times increased with increasing the equivalent particle diameter, but there was an important scattering in the results. The data dispersion greatly decreased when the shape factor of the wood particles was considered. The devolatilisation times were fitted to a power-law relation replacing the particle diameter by the equivalent particle diameter multiplied by the shape factor. The effect of the moisture content was studied by analysis of the devolatilisation process of pine wood particles of the same size and different moisture contents (0–50%). As the moisture content of the wood particles increased the devolatilisation rate of combustible volatiles decreased and was more uniform along the devolatilisation time.
The catalytic effect of pH-neutral inorganic salts on the pyrolysis temperature and on the product distribution was studied by fractionated pyrolysis followed by GC/MS and GC/FID and by thermogravimetric analysis (TGA) of cold-water-washed hornbeam wood. Sodium and potassium chloride have a remarkable effect on the pyrolysis temperature and on the product distribution, whereas calcium chloride only changes the low temperature degradation of hornbeam wood and the product distribution is nearly unchanged compared with water-washed hornbeam wood. All studied potassium salts (KCl, KHCO3, and K2SO4) decrease the amount of levoglucosan the order of magnitude being dependent on the anion: chloride has a more pronounced effect than sulphate, and sulphate a more pronounced effect than bicarbonate. The thermal degradation of three different wood species (hornbeam, walnut and scots pine) was investigated by analysis of thermogravimetric/mass spectrometric pyrolysis. Commonly used model substances for the main components of wood, like xylan, pure cellulose or filter pulp, were found to be unreliable for the evaluation of formal kinetic parameters that are able to describe the pyrolysis of wood. A method for the individual evaluation of formal kinetic parameters for the main components of wood was used, that uses specific ion fragments from lignin degradation products to study the lignin degradation. Coniferous lignin is thermally more stable than deciduous lignin, and the latter produces smaller char yields. The differences in wood species mainly result in different degradation rates for the lignin and for the early stages of the hemicellulose degradation.
A kinetic study of the pyrolysis of municipal solid waste (MSW) in a fluidized bed reactor was carried out. The MSW pellets are discharged onto the fluidized sand bed; in the upper part of the reactor the volatiles evolved from primary reactions underwent secondary cracking reactions. A correlation model was applied to simulate the primary and secondary reactions as well as the heat transfer process. The experimental yields of the total gases obtained in 49 runs performed at 700, 750, 800 and 850 °C fitted satisfactorily using a flexible simplex method. The values of the kinetic parameters of secondary reaction, the heat transfer coefficient from the bed to the sample and the ratio “secondary gas yield/primary tars yield” were optimised. The values of the secondary kinetic parameters, which showed a great inter-relation with those of the primary reactions, were within the range of the values proposed in literature for other biomass tar cracking.
A kinetically based prediction model for the production of organic liquids from the flash pyrolysis of biomass is proposed. Wood or other biomass is assumed to be decomposed according to two parallel reactions yielding liquid tar and ( gas + char) The tar is then assumed to further react by secondary homogeneous reactions to form mainly gas as a productThe model provides a very good agreement with the experimental results obtained using a pilot plant fluidized bed pyrolysis reactorThe proposed model is shown to be able to predict the organic liquid yield as a function of the operating parameters of the process, within the optimal conditions for maximizing the tar yields, and the reaction rate constants compare reasonably well with those reported in the literature
A novel moving and stirred bed reactor with a high heat transfer capacity has been operated to achieve the thermal decomposition of used tyre particles under vacuum. The overall heat transfer coefficient determined in this reactor reaches 200–250 W m−2K−1, a value exceeding the levels obtained in conventional rotary kilns and multiple hearth furnaces. In order to design large scale stirred bed vacuum pyrolysis reactors, both experimental and theoretical studies were carried out to understand the heat transfer mechanism and to determine the heat transfer coefficient in the reactor as a function of the operating conditions. In this work, the heat transfer coefficients under different agitation speeds up to 22.5 rpm were measured. The heat transfer coefficient was found to increase with the agitation speed, proportionally to (1/tmix)1/2. A Schliinder's modified model was used to describe the correlation between the heat transfer coefficient and the operating conditions. Calculation of the partial heat transfer coefficients during the three pyrolysis evolution periods revealed the influence of the chemical reactions, the phase change and the feedstock thermal property variation on the overall heat transfer coefficient during the vacuum pyrolysis of tyre particles.
Flash pyrolysis of wood in a circulating fluidized bed is studied. The results of a comprehensive gas-solid reaction model are used successfully in analysing the system. The changes in the structure, the transient nature of heat and mass transfer and the reaction scheme are accounted for. All the structural parameters and thermophysical properties are used as continuously changing variables.
The modelling of the spread of fire and its extinguishment still represents a significant challenge. As part of a combined experimental and computational study of fires we have developed a general Computational Fluid Dynamics (CFD) model of fire spread and extinguishment. The primary objective was to produce a flexible computational tool which can be used by engineers and scientists for design or research purposes. The present paper deals with the description and validation of a solid pyrolysis model which has been applied, as a sub-model, in this general computer fire code. The pyrolysis model has been formulated using the heat-balance integral method. The model can be applied to slabs of char forming solids, such as wood, as well as non-charring thermoplastic materials, such as PMMA. Results are compared with analytical solutions, numerical simulations and experimental data. In all cases the integral model performs well. © 1997 by John Wiley & Sons, Ltd.
A comprehensive mathematical model of wood pyrolysis, based on the unreacted-core-shrinking approximation, is used to assess the role played by the description of the kinetics for a one-step global reaction. The more accurate model, which was previously subjected to experimental validation, includes a first-order Arrhenius rate, whereas the simplified one uses the assumption of constant (assigned) temperature, T-p, at the pyrolysis front. Both models predict qualitatively similar particle dynamics. Extensive simulations, carried out by varying the parameters of the kinetic models, the external heating conditions, and the particle size, indicate that the unknown parameter T-p, though comprised in the range of experimental values, should not be chosen coincident with the pyrolysis temperatures predicted by the finite-rate model. However, a range of T-p values can be determined that produces chief process characteristics, such as maximum devolatilization rate and conversion time, very close to those of the finite-rate model. In this way, acceptable agreement is also obtained between predicted and measured integral and differential mass loss curves of thick wood exposed to radiative heating.
A new integral thermal pyrolysis model for the transient pyrolysis of charring and on-charring materials has been developed and evaluated by comparison of the results with exact solutions. The purpose for the development of this simple model has been the desire for predicting pyrolysis histories of materials exposed to pre heat fluxes by using “equivalent” properties tailored to the present model and common flammability test measurements. The pyrolyzing material is divided into a char layer and a unpyrolyzed (virgin) layer where the material has not yet pyrolyzed. These two layers are separated by an isothermal interface which is at a pyrolysis temperature (characteristic of the material). At this interface, heat is transferred to the virgin layer, causing further pyrolysis of the material (namely a thermal pyrolysis model is used). A one-dimensional transient heat conduction model is used to predict the heat transfer within the material. Exponential temperature profiles were assumed for the heat conduction model. Using a rwo-equation 9-moment method, the original partial differential equations were transformed into a set of two ordinary differential equations for each layer. These equations were numerically salved to I) determine the pyrolysis rate. regression depth and surface temperature, and 2) establish a dimensional and sensitivity analysis. The model has been shown to be very accurate (errors ~ 2%) from comparisons between numerical results and exact solutions. Despite the neglect of derailed chemical kinetic (Arrhenius) pyrolysis expressions, the accuracy of the integral model together with its simplicity has allowed the deduction of pyrolysis properties of materials by using common flammability test data as it ';ill be proved in a subsequent paper.
Fluoroptic temperature measurement has been applied to determine the external heat transfer coefficient of particles flowing along the surface of a rotating cone reactor, specified by a half cone top angle of 45° and a maximum diameter of 68 cm, which has been designed for flash pyrolysis of biomass. Two different hydrodynamic regimes have been considered. Both, the cooling of a very dilute stream of hot particles, flowing freely along the cold cone wall and the cooling of hot particles in a very dense cold sand flow (moving bed regime) were studied. In the very dilute regime (without sand supply), the derived heat transfer coefficients are in the range of 500–1000 W m−2 K−1 and display a minimum as a function of the cone rotation frequency. Experiments at cone rotation frequencies of 3.77–5.28 Hz show that heat transfer coefficients for small particles (average particle diameters of 159 and 284 μm) are reasonably well predicted by the correlation of Ranz and Marshall [W.E. Ranz, W.R. Marshall, Evaporation from drops: Part 2, Chem. Eng. Pr. 48 (1952) 173] for heat transfer by gas phase convection to a non-spinning sphere in free flight. Contrary, larger particles with an average diameter of 428 μm show significantly higher heat transfer coefficients than expected on basis of the Ranz and Marshall equation. This is explained by a changing flow pattern of the particles over the conical surface and the consequences for the slip velocity between gas phase and particles. Large deviations from the Ranz–Marshall equation at a cone rotation frequency of 3.01 Hz are explained in terms of an increased contact with the wall resulting in a higher contribution of conduction to the total heat transport. For sample particles in a flow of sand with an average diameter of 350 μm, the determined heat transfer coefficient gradually decreases as a function of the cone rotation frequency; it remains constant however for coarse sand (750 μm). These phenomena have been explained in terms of variation in density of the gas/solids emulsion.
In order to investigate vapour phase cracking of tar from pyrolysis of spruce wood, experiments with small particles of spruce wood (0.5–1.0mm) were carried out both in a thermogravimetric analyser (TGA) and in a coupling of the TGA with a consecutive tubular reactor. In all experiments, the TGA was heated from 105 to 1050°C at a heating rate of 5Kmin−1. The tubular reactor was operated at temperatures of 600, 700 and 800°C at different residence times, these being achieved by providing the reactor with three independent heating zones. For the tubular reactor, a one-dimensional flow model was employed which took axial disper